Asymmetric Digital Subscriber Line (ADSL) is a communication technology that enables existing twisted-pair telephone lines to serve as access paths for applications such as multi-media and high-speed data communications. An ADSL circuit includes an ADSL modem connected at each end of a twisted-pair telephone line. In one configuration, an ADSL service provides three information channels: a high-speed downstream data channel, a medium speed upstream data channel and a Plain Old Telephone Service (POTS) channel. The combination of simultaneous downstream and upstream data channels yields duplex data transmission as well as simultaneous POTS service.
In ADSL applications, often associated with the POTS channel are POTS signaling transients that primarily result from telephone ringing voltage cutting on and off, such as the changing back and forth between the active ringing interval and the silent ringing interval. A typical ringing cycle consists of a 2 second active ringing interval followed by a 4 second silent ringing interval, and then the overall ringing cycle repeats over and over again until the called party answers. Use of unbalanced ringing, which is prevalent in the USA, makes the effects of these signaling transients more severe than is the case for balanced ringing. The unbalanced ringing voltage, including its cut-on and cut-off transients, has a common mode voltage component and a differential voltage component. The ringing voltage changes rapidly when it cuts on and off and this is one type of POTS signaling transient. In the case of unbalanced ringing voltage, these signaling transients have both a common mode and differential transient voltage component.
ADSL modems are generally associated with a separate POTS low pass filter for filtering the differential transient voltage as well as for filtering the ADSL signal from getting through to the voice terminals. However, some conventional POTS low pass filters, such as those offered by Coming Cabling System, provide little or no filtering of the common mode component of the transient voltage. Furthermore, some of the unfiltered common mode transient voltage passes through the line isolation transformer of an ADSL transceiver unit (ATU).
A value of an encoded bit of information can be altered unintentionally in the presence of common mode transient voltage, causing the bit to be interpreted incorrectly by the ADSL transceiver unit. Such an unintentionally altered bit is commonly referred to as a bit error. The percentage of received bits in error compared to a total number of bits received over a period of time is commonly referred to as the bit error rate. In such a period of time, reducing the bit error rate increases the time available for transmitting ‘good’bits of information and minimizes the detrimental effects of the ‘bad’ bits of information. One of the main purposes of the POTS low pass filter is reduce the bit error rate that results from POTS signaling transients.
Another type of POTS signaling transient is the ring trip transient that occurs when the called party answers a call coming to him by taking his phone off hook. A phone has low impedance at DC and low frequencies when off hook and has a high impedance when it is on hook. Thus, a ring trip transient causes the current through the phone and the POTS low pass filter to increase significantly and abruptly.
The bit error rate associated with the ring trip transient is worse if the call is answered during a ringing interval rather than a silent interval of the ringing cycle. This is because a central office POTS switch applies only a DC voltage to the line during the silent interval. A higher amplitude AC ringing voltage is superimposed on the DC voltage and applied to the line during the ringing interval, and this causes the ring trip currents to be higher during the ringing interval than they are during the silent interval. Bit errors associated with an ADSL transceiver unit of the ADSL modem can be caused by the high ring trip currents in POTS low pass filter designs where the magnetic coils (or inductors) are not capable of handling the worst case ring trip current without experiencing DC current saturating. DC saturation current is an important factor in determining the size of the coils in a POTS low pass filter design.
There is a delay from when the called party answers until the central office POTS switch detects the phone going off-hook. The phone going off-hook causes the switch to cut off the AC ringing voltage being applied to the line and this completes the overall ring trip event. The cutting off of the ringing voltage at this time is a POTS signaling transient in itself and is similar to the POTS signaling transients discussed earlier that pertained to the ringing event prior to the ring trip.
In addition to limitations associated with ADSL bit error rate performance, the utility of conventional POTS low pass filters can also be limited by their physical size. Some conventional POTS low pass filters are too large for use on many high-density Digital Subscriber Line Access Multiplexor DCSLAM) units. In many instances, conventional POTS low pass filters include one or more magnetic coils whose physical size takes up too much volume for use with high-density DSLAM units
Therefore, a low pass filter including a circuit design that contributes to providing increased ADSL bit error rate performance in the presence of POTS signaling transients and that provides for a relatively small low pass filter package size is useful.